1. Filed of the Invention
The present invention relates to a connection structure for connecting the conductors of a flat cable to the terminals of a mating part (external part).
2. Description of the Related Art
FIG. 7 is a plan view showing a conventional flat cable and FIG. 8 is a plan view showing the portion where the conventional flat cable, a conventional lead block and conventional lead wires are connected, the flat cable 13 being a band-shaped member which carries conductors 15 composed of a copper foil or the like on an insulating base film 14 composed of polyethylene terephthalate (hereinafter referred to as PET) or the like with the conductors 15 partly exposed from an end of the base film 14. For example, as shown in FIG. 9, in a rotary connector 16 used as the conductor means of an airbag circuit mounted on an automobile, a flat cable 13 is accommodated in a pair of first and second housings 17, 18 which are rotatably connected to each other. Although both the ends of the flat cable 13 are directly or indirectly taken out to the outside of the housings 17, 18 by being made perpendicular to the lengthwise direction of the flat cable 13, there is known a connection structure for connecting the conductors 15 of the flat cable 13 to external lead wires 20 through the terminals 21 of a lead block 19 (refer to FIG. 8) as the latter indirectly taking-out means.
There has been conventionally proposed a method of connecting the conductors 15 of the flat cable 13 to the terminals 21 of the lead block 19 using ultrasonic welding.
The connecting method using the ultrasonic welding is arranged such that the terminals 21 of the lead block 19 and the conductors 15 of the flat cable 13 are placed on the anvil of an ultrasonic welder and ultrasonic vibration is applied to the portions where the terminals 21 overlap the conductors 15 in the state that the portions are pressed by a horn to thereby cause solid-state welding to the portions where the terminals 21 are connected to the conductors 15. This method has advantages such as a connecting job is simple and a working time can be shortened as compared with other connecting methods such as soldering, spot welding, etc. because a plurality of sets of the terminals 21 and the conductors 15 can be connected to the conductors 15 at a time.
Further, when the conductors 15 of the flat cable 13 are connected to the terminals 21 of the lead block 19, it is necessary to correctly determine the mutual positions of the conductors 15 and the terminals 21.
For this purpose, as shown in FIG. 7, there are conventionally formed a pair of cutout portions 14a, 14a on the side edges of the base film 14 in the vicinity of an end thereof using the positions of the conductors 15 exposed by themselves as a reference. This is because that the distances from the side edges of the base film 14 to the conductors 15 are inevitably dispersed in manufacture and the side edges of the base film 14 cannot be directly used as the reference. On the other hand, as shown in FIG. 8, a pair of projections 19a, 19a which correspond to the pair of cutout portions 14a, 14a are disposed upward to the exterior package of the main body of the lead block 19. The pair of cutout portions 14a, 14a of the flat cable 13 are engaged with the pair of projections 19a, 19a of the lead block 19 to thereby correctly determine the mutual positions of the terminals 21 of the lead block 19 and the conductors 15 of the flat cable 13.
Incidentally, there is recently a tendency that the conductor 15 of the flat cable 13 is made thinner and there appears the flat cable 13 having the conductor 15 formed to a very thin thickness of, for example, about 32 .mu.m. When the thin conductor 15 is placed on an ultrasonic welder for ultrasonic welding, since the strength thereof is not sufficient due to its thin thickness, the conductor 15 is liable to be twisted and a plurality of the conductors 15 are liable to be separated, whereby the press force of the horn does not uniformly act on the portions where the terminals 21 are connected to the conductors 15, the mutual positions of the terminals 21 and the conductors 15 are displace and the solid-state welding is not made to target positions.
In particular, when the plurality of conductors 15 of the flat cable 13 are simultaneously ultrasonic welded to the respective terminals 21 of the lead block 19 which correspond to the conductors 15, since the conductors 15 are separated, there is a problem that the determination of the positions of the respective conductors 15 to the terminals 21 becomes difficult and the connected states of the respective sets of the conductors 15 and the terminals 21 are liable to be dispersed and thus all the sets of them cannot be securely connected.
When the ultrasonic welding is executed, there is caused a phenomenon of so-called "scatter of metal" that the metal (copper foil piece) of the plurality of conductors 15 of the flat cable 13 is melted therefrom by ultrasonic vibration and scattered to the outside of the conductors 15. When metal particles are scattered to the outside of the conductors 15 by the phenomenon of the scatter of metal, there is a problem that the plurality of conductors 15 may be electrically short-circuited, whereby the reliability of the connected portions is made insufficient and further a desired electric performance cannot be obtained.
Further, when the plurality of conductors 15 of the flat cable 13 are connected to the respective terminals 21 of the lead block 19, the mutual positions thereof must be correctly determined. The mutual positions are determined by the engagement of the pair of cutout portions 14a, 14a disposed to the flat cable 13 with the pair of projections 19a, 19a disposed to the lead block 19. Although the pair of cutout portions 14a are formed using the positions of the conductors 15 exposed by themselves as the reference, since the conductors 15 are twisted or separated when they are thin, it is difficult to easily obtain the correct positions to be used as the reference. Thus, there is a problem that the positions of the pair of cutout portions 14a, 14a cannot be correctly set.